1. Field of the Invention
The present invention relates to a circuit that sets, at a constant value, the voltage, signals and so on that are used in a semiconductor device.
2. Description of the Related Art
In semiconductor devices having a reference-voltage generating circuit that generates a reference voltage, the reference voltage generally shows some dispersion due to manufacturing conditions of the semiconductor devices and individual semiconductor devices or chips. Therefore, the reference-voltage generating circuit has been equipped with a control circuit that controls the reference voltage using fuses or the like. Such a reference-voltage generating circuit is disclosed, for example, in Japanese Laid-open Patent Publication H1-117427 and is illustrated in FIG. 9.
In FIG. 9, reference numeral 93 is a control circuit that generates control signals S0 through S3, which are trimming output, depending on whether fuses are disconnected or not. Reference numeral 94 is a voltage divider circuit that selects, depending on the control signals S0 through S3, one of a plurality of voltages that are obtained by voltage division between two reference potentials VA and VB and that outputs the selected voltage to a node 51 as a reference voltage. Reference numeral 92 is a buffer circuit, which drives a load connected to its output terminal OUT so that the load voltage can become equal to the reference voltage input to the input terminal IN.
Further, a voltage divider circuit 94 consists of a divided-voltage generating circuit 194 and a selecting circuit 195. The divided-voltage generating circuit 194 consists of 2Nxe2x88x921 resistors Rj, where j=1, . . . , 15, connected in series between two reference electric potentials VA and VB and outputs divided voltages to nodes 1 through 16. The selecting circuit 195 receives output voltages from the divided-voltage generating circuit 194 and the control signals S0 through S3 and selects one of the divided voltages depending on the control signals that is to be outputted to the node 51 as a reference signal.
The selecting circuit 195 is composed of N-channel MOSFETs Q1 through Q16, Q101 through 108, Q111 through Q114, and Q121 and Q122, and buffers G01 through G04 that have complementary outputs so that the reference voltage can be determined by the control signals S0 through S3 with the closest voltages being distinguished by the Hamming code distance 1. The relationship between the control signals and the reference voltage at the node 51 is shown in Table 1.
In Table 1, 1 and 0 respectively represent a high level signal and a low level signal. The same applies hereinafter unless mentioned otherwise.
The control signals S1, S2, S3, and S0 can respectively adjust voltage by units of 1/15(VA-VB), 2/15(VA-VB), 4/15(VA-VB) and 8/15(VA-VB). Also, in the code comprising the set of the combinations of the values of the control signals S0 through S3, the Hamming distance between neighboring code words is 1. Therefore, the upper bits, which are the values of the control signals S3 and S2, are first determined by disconnecting fuses in order to determine a range of rough values of the reference voltage through a first measurement of the semiconductor device. Then, the value of the reference voltage can be determined by the lower bits, which are the values of the control signals S1 and S0, within a constant range through a second measurement of the semiconductor device.
More specifically, if the values of the control signals S3 and S2 are determined respectively as 0 and 1 by a first measurement, then the voltage between 6/15(VA-VB), 9/15(VA-VB) can be set by the control signals S1 and S0 through a second measurement.
We have shown control signals of 4 bits as a prior example. However, demands for adjustment of minute voltage values have increased in recent years. Therefore, there is a tendency for the control signals to be many bits through many trimming outputs. As a result, the places for fuse adjustment are increasing. Therefore, it requires much time to disconnect many fuses.
Further, an increase in the number of bits, which is the number of signal lines, brings an increase in the area of fuse circuits.
The object of the present invention is thus to solve the above problems and to reduce time for disconnecting fuses and the fuse circuit area.
To achieve the above object, the present invention provides a semiconductor device equipped with a control circuit that is connected to wiring lines that supply a predetermined voltage and outputs a first control signal depending on the predetermined voltage and a second control signal that can be set depending on whether a fuse is disconnected or not, a divided-voltage generating circuit that is connected between predetermined first and second potential points and outputs voltages between the two potentials, and a selecting circuit that selects as a reference voltage one of the voltages output from the divided-voltage generating circuit, depending on the control signals.
A semiconductor device in accordance with the present invention is equipped with a control circuit that is connected to wiring lines that supply a predetermined voltage and outputs a first control signal depending on the predetermined voltage and a second control signal that can be set depending on whether a fuse is disconnected or not. Therefore, the number of fuses can be reduced.
Preferably, the selecting circuit is constructed so that the Hamming distance between any code words of the control signals can be 1 for the two closest voltages and thereby continuous selection of voltage can be possible.
The selecting circuit may be constructed so that the code represented by the control signals can be the binary-coded decimal code. If constructed in this way, continuous selection of voltage within a constant range can be possible.
In a preferred embodiment, the control signals in the control circuit consist of a group of lower bits that can adjust minute voltage and a group of upper bits that can control voltage greater than the one that the group of lower bits can adjust. Then the second control signal may be the lower bit group to control fine adjustment of voltage with fuse circuits. In another method, the control signals of the control circuit also consist of a group of lower bits that can adjust minute voltage and a group of upper bits that can control voltages greater than the one that the group of lower bits can adjust. Then a first control signal may be one bit of the lower bit group and a second control signal may be one bit of the upper bit group.
The semiconductor device of the present invention may be equipped with fuse circuits that vary complementary first and second output signals depending on whether fuses are disconnected or not. In this case, if a constant voltage is at a central value, then the disconnection of fuses can be reduced at that central point.
The divided-voltage generating circuit described above may contain resistors of different resistances with constant ratios, and the resistors may be connected in series and the connections between them may be output terminals. In this case, the number of resistors and fuses may be reduced.
Further, a resistance means may be installed between the above divided-voltage generating circuit and the first or second potential point. In this case, a small amount of voltage can be given to the divided-voltage generating circuit, so that fine adjustment of voltage is made easy.
It is also preferable that the semiconductor circuit of the present invention has a voltage-control oscillator circuit that inputs a reference voltage to an oscillator circuit to control the oscillation frequency. By this means, the oscillation frequency can be controlled by a small number of fuses.